This invention pertains to microlithography (projection-transfer) in which a pattern, defined on a mask or reticle, is projected (and thus xe2x80x9ctransferredxe2x80x9d) to a suitable substrate using a charged particle beam such as an electron beam. This type of microlithography has especial utility in the fabrication of integrated circuits and displays. More particularly, the invention pertains to xe2x80x9creticle blanksxe2x80x9d from which reticles for charged-particle-beam (CPB) microlithography can be made, reticles made from such reticle blanks, and to methods for making such reticle blanks and reticles.
In recent years, as semiconductor integrated circuits have become increasingly miniaturized, the resolution limits of optical microlithography (i.e., projection-transfer of a pattern performed using ultraviolet light as an energy beam) have become increasingly apparent. As a result, considerable development effort currently is being expended to develop microlithography methods and apparatus that employ an alternative type of energy beam that offers prospects of better resolution than optical microlithography. For example, considerable effort has been directed to use of X-rays. However, a practical X-ray system has not yet been developed because of many technical problems with that technology. Another candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam.
A current type of electron-beam pattern-transfer system is an electron-beam inscribing system that literally xe2x80x9cdrawsxe2x80x9d a pattern on a substrate using an electron beam. In such a system, no reticle is used. Rather, the pattern is drawn line-by-line. These systems can form intricate patterns having features sized at 0.1 xcexcm or less because, inter alia, the electron beam itself can be focused down to a spot diameter of several nanometers. However, with such systems, the more intricate the pattern, the more focused the electron beam must be in order to draw the pattern satisfactorily. Also, drawing a pattern line-by-line requires large amounts of time; consequently, this technology has very little utility in the mass production of semiconductor wafers where xe2x80x9cthroughputxe2x80x9d (number of wafers processed per unit time) is an important consideration.
In view of the shortcomings in electron-beam drawing systems and methods, charged-particle-beam (CPB) projection-microlithography systems have been proposed in which a reticle defining the desired pattern is irradiated with a charged particle beam. The portion of the beam passing through the irradiated region of the reticle is xe2x80x9creducedxe2x80x9d (demagnified) as the image carried by the beam is projected onto a corresponding region of a wafer or other suitable substrate using a projection lens.
The reticle is generally of two types. One type is a scattering-membrane reticle 21 as shown in FIG. 3(a), in which pattern features are defined by scattering bodies 24 formed on a membrane 22 that is relatively transmissive to the beam. A second type is a scattering-stencil reticle 31 as shown in FIG. 3(b), in which pattern features are defined by beam-transmissive through-holes 34 in a membrane 32 that tends to scatter particles in the beam. The membrane 32 is normally silicon with a thickness of approximately 2 xcexcm.
Because, from a practical standpoint, an entire reticle pattern cannot be projected simultaneously onto a substrate using a charged particle beam, conventional CPB microlithography reticles are divided or segmented into multiple xe2x80x9csubfieldsxe2x80x9d 22a, 32a each defining a respective portion of the overall pattern. The subfields 22a, 32a are separated from one another on the membrane 22, 32 by boundary regions (items 25 in FIG. 3(a)) that do not define any pattern features. In order to provide the membrane 22, 32 with sufficient mechanical strength and rigidity, support struts 23, 33 extend from the boundary regions 25.
Each subfield 22a, 32a typically measures approximately 1-mm square. The subfields 22a, 32a are arrayed in columns and rows across the reticle 21, 31. For projection-exposure, the subfields 22a, 32a are illuminated in a step-wise manner by the charged particle beam (serving as an xe2x80x9cillumination beamxe2x80x9d). As the illumination beam passes through each subfield, the beam becomes xe2x80x9cpatternedxe2x80x9d according to the configuration of pattern elements in the subfield. As depicted in FIG. 3(c), the patterned beam propagates through a projection-optical system (not shown) to the sensitive substrate 27. (By xe2x80x9csensitivexe2x80x9d is meant that the substrate is coated on its upstream-facing surface with a material, termed a xe2x80x9cresist,xe2x80x9d that is imprintable with an image of the pattern as projected from the reticle.) The images of the subfields have respective locations on the substrate 27 in which the images are xe2x80x9cstitchedxe2x80x9d together (i.e., situated contiguously) in the proper order to form the entire pattern on the substrate.
Certain steps of a conventional method for fabricating a reticle are depicted in FIGS. 4(a)-4(d), respectively. In a first step, an xe2x80x9cSOIxe2x80x9d (Silicon On Insulator) substrate 40 is made from a silicon support substrate 41, a silicon oxide layer 42, and an SOI layer 43, using conventional fabrication techniques (FIG. 4(a)). The silicon oxide layer 42 is formed on a first major surface 50 of the silicon support substrate 41, and the SOI layer 43 is formed on the silicon oxide layer 42. In a second step, a dry-etching mask 46, made from a thick film of silicon oxide or resist, is formed on or applied to, respectively, the second major surface 51 of the silicon support substrate 41 (FIG. 4(b)). The dry-etching mask 46 defines openings 45 in which corresponding regions of the second major surface 51 are exposed. The silicon support substrate 41 exposed in the openings 45 is dry-etched (FIG. 4(c)).
The silicon support substrate 41 is dry-etched depthwise to the silicon oxide layer 42 that acts as an etch-stop barrier. In the resulting structure, the silicon oxide layer 42 and the SOI layer 43 are supported by a silicon peripheral frame 41b and silicon struts 41a contiguous with the peripheral frame 41b. Each strut 41a is approximately several hundreds of xcexcm wide. The struts 41a are spaced apart from one another so as to leave open areas between the peripheral frame 41b and the struts 41a, and between individual struts 41a. 
As evident in FIG. 4(c), the silicon oxide layer 42 is exposed in the openings between struts. The exposed silicon oxide 42 is removed using a mixture of HF+NH3F, leaving the SOI layer 43 to become a reticle membrane 43a (FIG. 4(d)), thereby completing manufacture of a reticle xe2x80x9cblank.xe2x80x9d
In the foregoing method, dry-etching must be performed to a depth equal to the thickness of the silicon support substrate 41 of the SOI substrate 40. For example, in the case of an SOI substrate 40 having a diameter of 3 inches, dry-etching is performed to a depth of at least 300 xcexcm. In the case of an SOI substrate 40 having a diameter of 8 inches, dry-etching is performed to a depth of at least 700 xcexcm.
Conventionally, if dry-etching is to be performed to a depth of at least 700 xcexcm, then a silicon oxide layer used as a dry-etching-mask desirably is at least approximately 10 xcexcm thick. (This is because the etching selectivity for silicon relative to silicon oxide is about 100:1. Hence, 700 xcexcm/100=7 xcexcm, which is approximately 10 xcexcm.) A resist layer used as a dry-etching mask conventionally is 50 xcexcm to 60 xcexcm thick. (The etching selectivity for silicon relative to resist is about 20:1. Hence, 700 xcexcm/20=35 xcexcm, which is approximately equal to 50 xcexcm.)
The dry-etching technique summarized above is performed according to the well-known sidewall-protecting plasma dry-etching technique. Sidewall-protecting plasma dry-etching protects the sidewalls of the cavities being formed by inhibiting the etching away of surrounding structure. The sidewalls are protected by flowing a mixture of the silicon-etching gas and a sidewall-protecting gas. Upon contacting the side walls, the sidewall-protecting gas in the mixture polymerizes and forms polymer deposits on the sidewalls. The polymer deposits tend to protect the sidewalls from further dry-etching so that dry-etching proceeds preferentially in the thickness direction while lateral dry-etching into the sidewalls is suppressed.
Examples of gas mixtures for performing dry-etching while protecting sidewalls are Cl2+CHF3, SF6+C3H8, and SF6+CCl4. These gas mixture from a protective film by polymerization of CHF3, C3H8, or CCl4, respectively. With polymer deposits on the sidewalls, dry-etching of the silicon tends to progress in the thickness direction by action of the Cl2 or SF6.
Conventionally, whenever a dry-etching mask 46 is made from a thick layer of silicon oxide, the underlying SOI substrate 40 tends to become distorted by stresses in the silicon oxide mask 46. Consequently, the silicon oxide mask 46 tends to exhibit peeling from the SOI substrate 40 as the mask 46 is being formed.
A dry-etching mask conventionally made solely from a layer of resist has a poor etching-selectivity ratio. To solve this problem, the resist layer is conventionally made several tens of xcexcm thick. However, since the resist itself also is etched during dry-etching (i.e., the resist has poor durability With respect to dry-etching), the edges of the openings in the resist recede (i.e., the openings in the resist enlarge) as dry-etching progresses, as shown in FIGS. 5(a)-5(b). Hence, by the time that dry-etching through the thickness of the silicon support substrate 41 is completed, the edges of the openings have receded (arrows in FIG. 5(b)) laterally beyond the sidewall-protective film. When dry-etching is performed under conditions in which the openings in the resist are enlarging, dry-etching occurs behind the sidewall-protective film into silicon that should be protected by the sidewall-protective film. As a result, icicle-like projections are formed along the sidewalls that are detached easily and create troublesome debris.
In view of the shortcomings of conventional methods, as summarized above, an object of the present invention is to provide methods for fabricating reticle blanks for charged-particle-beam (CPB) microlithography in which struts can be formed having sharply defined, vertical sidewalls, without the attendant formation of xe2x80x9ciciclesxe2x80x9d on or near the sidewalls.
According to a representative embodiment of a method, according to the invention, for manufacturing a reticle blank, a first step includes preparing an SOI substrate. The SOI substrate comprises a silicon support substrate and an etch-stop layer formed on a first major surface of the silicon support substrate. By way of example, the etch-stop layer is a silicon oxide layer and the membrane-forming layer is an SOI layer. A dry-etching mask is formed on a second major surface of the silicon support substrate. The dry-etching mask comprises a thin (desirably 1 xcexcm to 2 xcexcm thick) silicon oxide layer formed on the second major surface and a resist layer (desirably 20 xcexcm to 30 xcexcm thick) formed on the silicon oxide layer. The dry-etching mask defines a pattern of windows located at positions corresponding to spaces to be located between struts of the reticle blank. According to the pattern of windows in the dry-etching mask, the silicon support substrate is dry-etched. Afterward, exposed portions of the etch-stop layer and remaining portions of the dry-etching mask are removed to complete formation of the reticle blank. The reticle blank can be used to form a reticle.
As noted above, in manufacturing the reticle blank, a dry-etching mask is formed on the second major surface of the silicon support substrate side. The dry-etching mask is formed from a thin (desirably 1 xcexcm to 2 xcexcm thick) silicon oxide layer, formed on the second major surface, and a resist layer is formed on the silicon oxide layer. Even if the resist recedes during dry-etching, the dry-etching gas nevertheless will be blocked by the underlying silicon oxide layer from forming xe2x80x9ciciclesxe2x80x9d on side walls of recesses formed in the silicon support substrate. Consequently, if dry-etching is performed under conditions in which a sidewall-protective film is formed, the sidewall surfaces intended to be protected by the protective film are not dry-etched, thereby preventing the formation of xe2x80x9ciciclesxe2x80x9d on or near the sidewalls and preventing formation of debris.
In addition, because the silicon oxide layer in the dry-etching mask is thin (1 xcexcm to 2 xcexcm) compared to conventional practice (at least 10 xcexcm thick), the silicon oxide layer imparts very little stress to the silicon support substrate.
The foregoing and additional aspects, features, and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.